A conventional built-up roof structure utilizes a water-impermeable roofing layer disposed over and supported by a roof deck which may consist of a corrugated steel deck supported by structural I-beams or a structural plywood or composite board base supported on top of wooden or steel beams. Usually the water-impermeable layer consists of several layers of felt laminated together with coatings of bitumen. A protective layer of gravel generally is deposited over the water-impermeable layer to hold the water-impermeable layer in place. In geographical regions in which the roof is subjected to significant climatic temperature variations, it often is desirable that the roof be provided with insulation to reduce loss of heat through the roof when the ambient outside temperature drops and to reduce heat penetration through the roof to the interior of the building when the ambient temperature outside of the building appreciably exceeds the normal temperature within the building.
In U.S. Pat. No. 3,411,256, a built-up insulated roof construction is shown that includes flat panels of closed-cell thermal insulation foam adhered to the upper face of a water-impermeable membrane (usually formed of alternating layers of felt and bituminous material, although sheets of water-impermeable plastic may also be used). A protective layer of gravel is spread over the insulation panels to protect the insulation foam from UV (ultraviolet radiation) degradation and to aid in preventing the panels from being dislodged by winds. If desired, a fire-resistant layer is interposed between the roof deck and the water-impermeable membrane (such as described in U.S. Pat. No. 3,763,614) to provide a degree of fire protection for the roof structure in the event of fire.
Although the water-impermeable barrier layer and thermal insulation layer of the above-described built-up roof constructions are impervious to water, the materials from which these layers are formed allow moisture (water vapor) to penetrate into and through the layers. The ability of moisture to move through the water-impermeable barrier layer and the thermal insulation layer of the roof structure is desirable to reduce problems associated with moisture build-up within the roofing system, particularly when the ambient outside temperature is significantly below the temperature at the interface of the insulation layer and the water-impermeable barrier layer.
It will be appreciated that during a rain or as snow or ice melts on the built-up roof during a thaw, a film of water will form on the flat upper surface of the insulation panels of the roof. The water film interfers with the normal transfer of moisture through the insulation panel by elimination of the normal movement of moisture vapor from the more saturated regions at the bottom of the insulation panel to the less saturated regions at the top of the insulation with resultant evaporation of the moisture into the atmosphere from the panel's upper surface. As a consequence, an objectionable accummulation of moisture within the insulation panel can occur which reduces the thermal insulating properties of the panel. Also, when the temperature of the outside atmosphere drops below freezing, frosting and even freezing of moisture can occur within the insulation panel creating internal stresses within the panel that can in time cause a premature failure of the panel.